Thanks again @Steven Merrill for your help with this tool this past Sunday. I had another look at 7.x-2.0-rc3 and I think drush vagrant-alias-config should do the same thing as drush vagrant in 7.x-1.0-alpha4. However,

when using 7.x-2.0-rc3: 

~/kalastack (3.x)$ drush vagrant-alias-config
The following errors occurred when running "drush vagrant-list --vms":                                     [error]
PROJECTS_PATH is unknown or does not exist. Have you built a vm already? Try drush vagrant-build           [error]
Invalid argument supplied for foreach() list.vagrant.inc:24                                                [warning]
Drush command terminated abnormally due to an unrecoverable error.

when using 7.x-1.0-alpha4

~/kalastack (3.x)$ drush vagrant
Attempting to read the Vagrant SSH configuration.
No errors found in the Vagrant SSH configuration.                                                          [ok]
Please enter the information about the Drupal site inside the VM.
Alias [drush-vagrant]: my_alias
...

What happens when I drush vagrant-build? My vagrant is working fine. Thanks.